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The siren song of the free lunch, the gratis timeshare weekend, the complimentary hotel happy-hour can be incredibly alluring, but each often comes with unsavory strings attached — aged buffet fare, a halitosis-scented sales pitch, watered-down well-brand booze. But the internal-combustion-powered automobile is rife with opportunities for free (or really cheap) energy harvesting that scientific advances are poised to begin capitalizing on. You may know that 60-75 percent of the chemical energy in the fuel you burn wafts right out the tailpipe and radiator as waste heat. BMW has proposed using this heat to power a two-stage steam turbine capable of boosting fuel economy by 15 percent (Turbosteamer, Technologue June, 2006), but the mass and cost of the plumbing and hardware have yet to shrink to production-viable levels. Now some savvier new ways of harvesting heat — and even vibration — are coming to light.

The first employs shape-memory alloys of titanium and nickel (NiTinol) to drive a heat engine that spins a generator. Remember NiTinol from my June, 2007 column? It can be bent or stretched when cold and then returns to its original shape when heated. The simplified concept of the SMA heat engine dates back three decades and works like a bike chain drive, with the chain made of SMA wire. Imagine the chain bathed in cool water on the bottom, and heated by hot air on top. As the top of the chain constricts and the bottom stretches, the rear sprocket is pulled along. Make these sprockets slightly different diameters, force them to turn at the same speed and the expanding and contracting NiTinol does the pedaling. General Motors was recently awarded a $2.7 million grant from the Department of Energy to develop the technology.

Project leader Jan Aase envisions such a gizmo replacing a vehicle’s alternator, powering all the onboard electric systems, boosting fuel economy by 11 percent. Still to be determined is whether air or fluid cooling would be required to provide the minimum 15-degree temperature differential for the low-hysteresis alloys the team is working with. Their goal is to prove the technology viable within two years for possible production applications within five years. He admits it’s far less powerful than BMW‘s Turbosteamer, but boasts ten times the power density per pound. It probably will cost less too. The DOE likes SMA’s potential to generate power on furnace flues too.

Meanwhile, Duke University engineers are at work on piezoelectric devices mounted to cantilevers that can generate electricity from vibration. You’ve heard of piezo fuel injector pucks expanding to open a fuel nozzle when electricity is applied? These piezoelectric laminates produce electricity when they’re bent back and forth at a certain resonant frequency. In the past, that resonant frequency, or sweet-spot in the vibration spectrum has been too narrow to be of much use. But the Duke folks discovered that by putting a magnet on the end of the cantilever oscillating between two other magnets that can be moved in or out along the cantilever, they can greatly expand the range of resonant frequencies at which the piezo materials produce electricity.

Duke’s Brian Mann says engine and suspension vibrations are ripe for harvesting, and while the milliwatts or 100s of milliwatts of piezo-power these things produce won’t extend hybrid range meaningfully, it could allow wireless powertrain/chassis sensors to be installed in hard-to-access locations (like inside tires) or elsewhere without the need for complex wiring harnesses. And hey, in a 35.5-mpg world, every milliwatt counts.

But wait, there’s more power4free:

How exactly does GM envision packaging this gizmo on the exhaust system? Well, instead of a chain with slightly different size sprockets, the system packages a series of differently sized cylinders, some of which (the red ones in the diagram) are smaller and are pressed against the hot exhaust pipe; the others (blue in the diagram) are slightly larger and have cooling air (or fluid) flowing through them. Many loops of the SMA wire are wrapped around these cylinders in a sinusoidal fashion, pressed between the hot cylinders and the exhaust pipe, running around the outside of the cool cylinders. GM Lab Group Manager Nancy Johnson explains “Diameters of the cylinders are chosen to provide sufficient time length of exposure to the alternating hot and cold sections of the wire for phase transformation to occur in these localized sections with accompanying shape memory and stiffness changes. The continuous loop SMA wire elements are installed such that the starting stress within them at cold state is sufficient to produce pseudoplastic strain of appropriate recoverable magnitude. Once the exhaust pipe starts to heat up the SMA wire sections in contact with the hot cylinders start to recover strain through the shape memory [phenomenon]. This is sufficient to get the system spinning energetically. Appropriate packaging then allows this spinning motion to be used to generate electricity.”

Previous attempts at SMA heat engines have run into trouble generating enough friction with just the NiTinol wire to power the generator, and of course making the entire system weather and wear resistant enough for automotive application in the harsh underbody environment will present significant challenges. But we wish GM the best of luck bringing this (nearly) free lunch to the table.